Argos have a Huge Baby Event with over 1000 items at the moment with this offer included. Thought this a really good price for this combo. Great reviews for both too
The cot was £52.99 & is now £42.99, the mattress was £29.99 & is now £22.49 but if you scroll down the page to special offers you can get the mattress for half price ie £11.24 when added with the cot, so £54.23 for the combo package. You can even C+C for Free as this cot seems to be available instore as well as for delivery.
There are other cots also on offer that can have the mattress added for half price too, will pop links in 1st post.

This beech cot from BabyStart has two adjustable mattress positions, allowing you to change the height as your child grows.
2 position adjustable mattress base - allows you to lower the height of the mattress as your baby learns to sit and stand.
Cot specifications:
Size L125, W65, H79cm.
Weight 11.7kg.

BabyStart bring you a quality, hypoallergenic foam mattress for your little one. For convenience it is wipe-clean and washable yet also allows air to pass through the material. A brilliant mattress to help your child have a more comfortable nights sleep.

Suitable for cots.
Hypoallergenic foam mattress.
Removable washable cover.
General information:

Size L120, W60, D7.5cm